A communication network includes paths of nodes that route packets through the network. Optical amplifiers perform an important function within these networks by amplifying an optical signal in order to increase the operational length of an optical network. In some configurations, the efficiency of an optical communication network may be compromised by an effect known as polarization hole burning.
Optical communication systems designed to operate over long distances may suffer from multiple polarization-dependent effects that reduce the operational efficiency of the system. Polarization hole burning (PHB) is one of these phenomena. PHB may seriously reduce the performance of rare-earth doped fiber optical amplifiers, such as an erbium doped fiber amplifier (EDFA), used to amplify signal strength within the communication system.
PHB occurs when a strong, polarized optical signal is launched into an EDFA. This strong signal can cause anisotropic saturation of the amplifier. This saturation effect, which is related to the population inversion dynamics of the EDFA, depresses the gain of the EDFA for light with the same state of polarization (SOP) as the saturating signal. Thus, PHB causes a signal having a SOP orthogonal to the saturating signal to have a gain greater than that of the saturating signal.
As a result, amplified spontaneous emission (ASE) noise in the SOP orthogonal to the saturating signal may accumulate faster than in the SOP of the saturating signal. In a communication system utilizing a chain of EDFAs operating at or near saturation, ASE noise may accumulate at each amplifier stage. As the noise builds up over the course of the system, the signal-to-noise ratio (SNR) for a signal with a SOP orthogonal to the saturating signal may rise to unacceptable levels. The SNR in such cases can then cause errors in the received data stream. Accordingly, mitigating the effects of PHB in amplified optical systems is desirable.
One of the causes of the undesirable PHB effect is operating an EDFA in a way that leads to gain compression. Gain compression (“Cp”) is a measure of the difference of the amplifier's non-saturated gain (“Go,” or the gain when operating on a low power signal) and the amplifier's saturated operating gain (“G”). The operating gain, in decibels, can be measured by taking the difference between the saturated output power (“So”) and the input power of a saturating signal (“Si)”, as follows:G=So−Si. 
The corresponding gain compression may be calculated as the difference between the non-saturated gain and the saturated operating gain:Cp=Go−G. 
The gain in the SOP orthogonal to a saturating signal may be measured using a probe signal with an input signal orthogonal to the saturating signal by measuring the input power (“Pi”) and output power (“Po”) of the probe signal:Po−Pi=G+ΔG. 
The “ΔG” in the above formula represents the amount of PHB in the SOP orthogonal to the saturation signal. This is a result of operating the amplifier with a saturating signal. As gain compression of an amplifier increases, so does the amount and effect of PHB. For instance, a single EDFA operating at a gain compression of about 3 dB may produce a PHB of about 0.08 dB. However, when that EDFA operates in a more saturated condition, with Cp=9-10 dB, the PHB may rise to about 0.2 dB.
The degree of PHB may also be affected by other factors, such as the degree of polarization of the saturating signal. If a signal's SOP varies over time, the effects of PHB may be reduced.
While the degree of PHB may be small for a single EDFA, these effects may be seriously compounded in communication systems that chain together a series of EDFAs. A number of arrangements have been proposed for reducing the effects of PHB in optical communication systems. However, such arrangements continue to suffer from drawbacks such as an inability to deal with arbitrary channel loading, expense and difficulty of implementation, and the innate stability characteristics of rare-earth doped fiber amplifiers.